Electrical machines, especially rotating electrical machines, contain an electrical winding within a laminated core. The winding consists of electrical conductors (possibly already provided with primary insulation) and solid insulating materials as main insulation. Without further measures, there is no intimate connection between the laminated core, the conductors, and the main insulation, meaning that gaps and voids are formed. On operation under atmospheric conditions, these regions would be filled with air. Particularly with applications in the high-voltage range, this is unallowable, since partial electrical discharges would destroy the insulation within a very short time. This leads to the failure of the electrical machine.
In order to produce an intimate connection and hence to displace air, the winding is impregnated using a curable impregnating resin or impregnating varnish. The solid insulating materials here may be porous in design, to increase the uptake of impregnating resin. Examples of such systems are mica tapes, insulating papers or nonwoven fabrics. For the impregnation of high-voltage machines, mixtures of epoxy resins and liquid, cycloaliphatic acid anhydrides are state of the art—see, for example, U.S. Pat. No. 4,113,791 (A). The acid anhydrides here serve as curing agents for polyaddition with an epoxy resin and at the same time lower the viscosity, to the benefit of rapid and complete impregnation.
Acid anhydrides, however, are generally compounds with a sensitizing effect, especially if taken up via the respiratory tract. For this reason, corresponding safety measures are mandatory when operating with acid anhydrides.